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While procrastinatingworking, I am wondering whether it is optimal to use softened (or deionized) water for brewing coffee, rather than tap water which is in my area rather hard.

My gut feeling is that extraction of the coffee beans' aromatic molecules will be made more efficient if there are no ions on the water, but is this backed up by science?

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    $\begingroup$ I realize there is already an unanswered question about tea brewing, but there are large differences between tea brewing and coffee percolation I believe… $\endgroup$ – F'x Aug 8 '16 at 8:32
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    $\begingroup$ Related: Does the hardness of water matter when making coffee? $\endgroup$ – Loong Aug 8 '16 at 10:38
  • $\begingroup$ There can be to much of a good thing. Where I live the water has too many ions in it making for terrible water. It is technically brackish. You can easily test this for a rough idea of where your water is by doing a total dissolved solids (TDS) test. I required a reverse osmosis system, after the water softener to get my tap water to a good baseline quality, Brita filters and the like do not lower the TDS. The RO system was money well spent for my water quality and taste, but test first. You can also call the water company to ask what additives they add so you know how to deal with them. $\endgroup$ – brose Jan 23 '17 at 12:49
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According to a discussion I had on this matter with a friend in the coffee processing industry, you should definitely consider yourself lucky for your naturally hard water. We live in a region with very soft water and he pre-treats all of his coffee-making water with a bed of ground granite (as a physical substrate) and powdered gypsum $\ce{(CaSO4 *2H2O)}$. For individual use he suggests mineral water, defined in Wikipedia as:

"The U.S. Food and Drug Administration classifies mineral water as water containing at least 250 parts per million total dissolved solids (TDS), originating from a geologically and physically protected underground water source."

The "taste testing" conferences attended by my friend have rigorously determined that DI water gives the "blandest" taste and water high in calcium and sulfur containing compounds to be the best.

Unfortunately, the chemical basis for this is poorly understood. It could certainly be pH related, and that my friend's treatment bed just raises the pH which in turn better extracts the desirable components of coffee relative to the undesirable. The same could be true of the mineral water and hard water in general should have a higher pH than soft water.

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